The present invention enables use from the lower part of the spectral zone to 1 GHz in the liberated upper part of the UHF (Ultra High Frequency) In fact this zone was previously used for broadcasting television in analogue. The passage of television broadcasting from analogue to digital has thus liberated this zone. These frequencies are dedicated to audiovisual broadcasters to implement High Definition (HD) broadcast services and to mobile operators for the purpose of launching access services to the Internet via cell networks. Their positioning in the UHF spectrum at less than 1 GHz ensures a long range and good penetration in buildings.
But the programmed appearance of Internet access services in the upper part of the UHF spectrum clearly poses the problem of cohabitation in terms of interferences with the DVB_T or DVB_H standard reception services. In fact it is recommended to assign the entirety of the sub-band 790-862 MHz, or 9 channels, to electronic communications.
In addition the miniaturized portable items of equipment are multiband and multimode and have a number of important wireless functions.
Extra flat terminals thus integrating an antenna inside their cover of which some cover 4 to 5 frequency bands and wide band antennas generally have mediocre levels of performance and remain relatively bulky whereas quality antennas are small but of narrow band.
The use of the UHF band located from 470 to 862 MHz renders the design of miniature antennas more delicate. It thus involves electrically small antennas with physical dimensions very much smaller than the wavelength.
As a consequence the performances in terms of efficiency are mediocre. The efficiency is in addition particularly dependent on the influence of the ground plane, the coupling, the positioning within a large number of different circuits and the impedance matching.
A circuit and a method for wideband impedance matching electrically small antennas is known from the document U.S. Pat. No. 6,121,940.
This impedance matching circuit is formed by active circuits simulating the behaviour of a non-Foster reactance such that the circuits at negative capacities and at negative inductances to obtain a wideband matching of electrically small antennas. This impedance matching circuit enables the equivalent reactance of the antenna to be neutralised.
However this circuit relates to frequencies of 30-88 MHz for dipoles of length of approximately 1 meter and is not adapted to the frequencies used by the invention and corresponding to the UHF channels.
In order to realise non-Foster reactance, and active circuit is used that from a positive impedance, capacitive or inductive, will realise its image impedance. This type of structure is referred to as a negative impedance converter.
The principle of a negative impedance converter is described in relation to FIG. 1. Two transistors T1 and T2 are mounted differentially in such a way that the base of the first transistor T1 is connected to the ground plane by the intermediary of a resistance R2 and such that the base of the second transistor T2 is connected to a capacitive element C2.
To compensate for the antenna reactance it suffices therefore to position this converter in series with the antenna, that is to say that the transmitter of the second transistor T2 is connected to port 1 of the antenna and that the transmitter of the first transistor is connected to port 2 of the receiver, so as to obtain a wideband impedance matching.
In the case of small antennas in UHF band, the refection coefficient of the antenna shows that the reactance varies for example from 0.6 pF to 3.5 pF in the UHF band, the results is that a perfect compensation of the antenna reactance would require the same variation law to be followed.
However this concept known as the Linvill concept is only valid at low frequency.
On the other hand if this differential structure at high frequency is analysed, it is noted that it is similar to those used to realise negative resistances in silicon technology and results in the compensation viewed from the receiver side is tainted with an additional positive resistance.
In addition this circuit being positioned directly after the antenna it is important that it does not introduce attenuation that would penalise the sensitivity or the gain of such a structure.
Thus if a gain of 1 GHz must be ensured, the cut-off frequency defined by the resistance R2 and the parasite capacity of the collector of transistor T2 must be greater than 1 GHz.